1. Technical Field
Embodiments of the present invention are directed to an electromagnetic shielding structure, and in particular, to a structure configured to be mounted to a substrate over a microelectromechanical system device to shield the device from electromagnetic interference.
2. Description of the Related Art
Microelectromechanical systems (MEMS) are mechanical devices formed using processes originally developed for manufacturing integrated circuits and the like. In the last decade, MEMS devices have become increasingly more diverse and complex, finding use in more and more consumer and industrial applications. A partial list of MEMS devices in common use includes accelerometers, gyroscopes, inclinometers, optical switches, fluid pumps, biological testing devices, pressure sensors, motors for aligning read/write heads for hard drives, vehicle airbag triggers, microphones, thermometers, etc. As MEMS devices become more sophisticated and sensitive, in many cases they have become susceptible to electromagnetic interference (EMI) generated by nearby circuits or other sources. This is especially true for sensors of various types, such as, e.g., microphones. To protect such devices from EMI, shield caps are sometimes provided, which are placed over MEMS devices to intercept and shunt to ground EM radiation that might otherwise affect the effectiveness of the particular device.
FIG. 1 is a cross-sectional side view of a MEMS EMI shield cap 50 according to known art. Typically, the such devices are made from laminated sheets of Bismaleimide Triazine (BT). The shield cap 50 of FIG. 1 has a first BT layer 52 in which an aperture 60 is formed, and a second BT layer 54 that closes the aperture 60, forming a blind aperture, or cavity. The facing surfaces of the first and second layers 52, 54 have respective layers of copper foil 56, 58. A thin adhesive layer 62 bonds the facing surfaces together. The interior of the cavity 60 and the bottom surface of the first BT layer 52 (as oriented in FIG. 1) are plated in layers of copper 64, nickel 66, and gold 68.
A conductive adhesive is used on the bottom surface of the first layer 52 to mechanically couple the shield cap 50 over a MEMS device on a substrate, and electrically couple the conductive lining of the cap to a circuit ground. EM radiation cannot penetrate conductive plating in the shield cap, but is instead carried to ground by the device.
The shield cap 50 is manufactured as one of hundreds that are made in sheets or wafers then cut into individual caps. The manufacturing process includes depositing a seed layer of copper on the interior of the cavity 60 and the bottom surface of the first BT layer 52, then electroplating, in succession, copper, nickel, and gold layers 64, 66, 68.
The initial seed layer does not adhere to the adhesive layer 62 at A, but, provided the adhesive layer is sufficiently thin, the build-up of the copper layer 64 on the side wall surface 70 and back wall surface 72 of the cavity 60 will bridge the gap to form a continuous plated surface. This is important because if the plating on the back wall surface 72 is not electrically coupled to the plating on the side walls 70, EMI that strikes the back wall will not be shunted to ground, but will instead be reradiated inside the shield cap 50, which would, of course, defeat the purpose.